The following is a discussion of the relevant art, none of which is admitted to be prior art to the appended claims.
Epidemiological studies in the U.S.A. and abroad have shown associations between mortality and morbidity and human exposure to ambient particulate matter (Schartz and Dockery, Am. Rev. Resp. Dis. 145:600, 1992; Pope et al., Am. Rev. Resp. Dis. 144:668, 1992). To date, there is limited knowledge about physical or chemical property of particulate matter that are responsible for these health effects and there is an increasing interest in developing accurate measurements in the near future.
The U.S. EPA has recently recognized the need to develop continuous measurement techniques for inhalable particulate matter (PM.sub.10 and PM.sub.2.5). Both individual activity patterns and concentration profiles of particulate pollutants vary for time scales much shorter than 24 hours. Thus realistic exposure assessment requires sample collection over time intervals about one hour or less.
The majority of the current particulate mass measurement methods use a size selective inlet to remove particles above a certain size, usually 10 .mu.m in diameter (PM.sub.10) Most of the available data on PM.sub.10 and PM.sub.2.5, have been obtained using gravimetric methods. The collected particles, usually on Teflon filters, are weighed using microbalances under constant specified temperature and relative humidity conditions. However, gravimetric methods are not sensitive enough to measure samples for durations of less than 24 hours.
The Tapered Element Oscillating Microbalance (TEOM.RTM.) is a recently developed method that originally appeared to be very promising (Patashnick and Rupprecht, "Continuous PM-10 Measurements Using the Tapered Element Oscillating Microbalance" J. Air Waste Manage. Assoc. 41:1079, 1991). According to this method, the air sample is heated up to 50.degree. C. to remove moisture, and particles are subsequently collected on a TEFLON filter that oscillates at the top of a metal rod. The amplitude of the oscillation decreases as the mass of the particles collected on the filter increases. Although this method is highly sensitive, its measurements are subject to a number of interferences; significant losses occur for semivolatile organic and inorganic compounds that in some areas can represent relatively large fraction of the total particulate matter. This problem is more pronounced for PM.sub.2.5, which includes unstable compounds such as ammonium nitrate and carbonaceous aerosols. For areas such as California and large urban environments, this method would significantly underestimate particle mass concentrations. Also, as the composition of the air sample changes, the partitioning of air pollutants between the gas and particle phase changes, therefore absorption and/or desorption processes can take place on the filter (depending on whether the air sample becomes more or less polluted). Due to the sensitivity of the method, these phenomena can cause either negative or positive artifacts. The gains and losses of mass on the filter are a serious problem, not just of the TEOM.RTM., but of any method that collects particles on a filter over a prolonged period of time (on the order of days). In the case of the TEOM.RTM., the filter media are usually exposed for a week. Finally, this method presents oscillations in its response which cancel out if a large number of measurements are added to determine a multi-hour concentration estimate; however, over shorter time intervals the measurement errors due to this oscillation can exceed 20-30%.
In addition, short-term measurement of particle size distributions is at least as important as short-term measurement of total particle mass concentrations. In fact, particle size may be the most important particle parameter, since the majority of the physical processes governing the behavior of particles depend on particle size. The sources, formation mechanisms, chemical composition as well as lifetime of ambient articles greatly change with particle size. Moreover, the uptake, retention and clearance of particles by the human respiratory system depends on the particle size. Thus obtaining short-term measurements of the size distribution of ambient particles, particularly those in the accumulation mode (i.e., with aerodynamic diameters smaller than 2.5 .mu.m) could substantially improve exposure assessment to particles and thus environmental decision making.
To date there is no adequate monitoring technique that determines the size distribution of ambient particles based on mass in short time periods. Quartz crystal piezobalances determine particulate mass indirectly through particle impaction on an oscillating quartz surface (Lundgren, D. A. In Fine Particles, edited by B. Y. H. Liu, Academic Press Inc., New, York, 1976; Chuan, R. L., In Fine Particles, edited by B. Y. H. Liu, Academic Press Inc., New, York, 1976). A quartz disk oscillates in an electric circuit at a highly stable resonant frequency which is inversely proportional to the particulate mass impacting and adhering onto the sensor. Such instruments suffer from the following potential shortcomings. First, the relationship between frequency and mass becomes non-linear for high particulate loadings. Second, since particles are collected on the crystal by impaction, the instrument response will be dependent on the sharpness of the collection efficiency and the extent of particle bounce and internal particle losses. Finally, aerosols consisting of carbonaceous particles which are composed of long stable chains of very small primary particles, cannot be determined with piezobalances. The chain aggregates contact the sensor at 2 to 3 points with most of the particulate mass waving above the sensor surface (Lundgren, D. A. and Daley, P. S., Am. Ind. Hyg. Assoc. J., 581-588, 1977).
Other direct-reading methods to determine particle concentration and size distribution include optical and electrical counters. Most of the optical systems count light pulses scattered from particles that flow, one by one, through an intensely illuminated zone. One limitation is the dependence of the instrument's response on the particle refractive index (and consequently on particle composition). In addition, the smallest detectable particle size is about 0.3 .mu.m, while much of the fine ambient particulate mass is due to particles smaller than this size. The Aerodynamic Particle Sizer (APS) (Model 3310, TSI Inc., St. Paul, Minn.; Wilson, J. C. and Liu, B. Y. H., J. Aersol. Sci. 11:139-150 1980; Baron, P., Aerosol Sci. and Technol. 5:56-67, 1985), sizes and counts particles by measuring their time-of-flight in an accelerating flow field. Particle measurement is based on particulate inertia, hence the system determines the aerodynamic particle diameter. The main shortcoming of the APS is that it cannot determine size for particles smaller than about 0.7 .mu.m.
Electrical counters have been used to determine particle size, based on charging the sampled aerosols and measuring the ability of particles to traverse an electrical field. The most widely used instrument of this type is the Differential Mobility Analyzer (DMA) (Model 3932, TSI Inc., St. Paul, Minn.). This technology is limited to measuring ambient aerosols in the size range 0.01-0.5 .mu.m. Using the DMA in conjunction with an optical counter or the APS would make it possible to determine a broad size range of atmospheric particles. Nevertheless, there are still three other shortcomings. First, both optical and electrical counters determine the number size distribution of particles which they subsequently convert to volume distribution. Since the density of ambient particles varies significantly (in the range of .+-.30% of the mean value), and since mass concentration is directly proportional to the density, large uncertainties can result from using these methods to determine particle mass concentrations as a function of size. Second, these techniques require conversion of the size distribution, by number, to a corresponding volume size distribution. The size distribution, by number, of ambient particles is dominated by ultrafine particles (i.e., smaller than 0.1 .mu.m). The coarser the particles, the smaller their number concentration becomes. However, when converting a number to volume distribution, a 1.0 .mu.m particle weighs as much as 10.sup.3 of 0.1 .mu.m particles and 10.sup.6 of 0.01 .mu.m particles. Consequently, counting errors (which are substantial for large particles, due to their relatively low number concentrations combined with electronic noise) associated with this conversion method are bound to lead to significant uncertainties in volume and consequently mass as a function of particle size. Finally, these instruments are very expensive (the combined optical/electrical counter cost is up to $100,000), with high maintenance costs, and thus are not suitable for large-scale field studies.
The U.S. EPA also recognizes the need to develop continuous measurement techniques for particle-bound water. Accurate measurement of particle-bound water is of paramount importance to the field of atmospheric chemistry, since hygroscopic ambient particles can be the media for a number of important homogeneous aqueous phase reactions. Moreover, particle water content affects particle-light interactions, and is therefore essential information for understanding and modeling visibility reduction.
To date there are no adequate monitoring techniques that measure particle-bound water. Existing techniques such as the Tandem Differential Mobility Analyzer (TDMA) (McMurry, P. H., and Stolzenburg, M. R., Atmos. Environ. 23:497-507, 1989) can only provide qualitative information. The TDMA method is based on the measurement of the particle size distribution before and after drying the particle sample. However, this method uses "calculations of ion strengths and molalities," based on "laboratory derived thermodynamic data for aqueous solutions of pure species." Since actual ambient particles contain variable mixtures of the different species, these calculations are imprecise. Also, most of the water is associated with particles above 0.5 microns in diameter, while DMA measurements are only accurate below this size. In addition, this method is not very sensitive because relatively large amounts of bound water can correspond to small changes in particle size due to the dependence of mass on the third power of the radius.
Direct-reading particle mass measurement methods such as the Quartz Crystal Piezoelectric Balance (QCPB) (Lundgren, D. A. In Fine Particles, edited by B. Y. H. Liu, Academic Press Inc., New, York, 1976) and the Tapered Element Oscillating Microbalance (TEOM.RTM.) (Pataschnick, H., and Rupprecht, E. G., JAWMA, 41:1079-1083, 1991), could be used to measure particle-bound water. However, these methods present some serious limitations. In the case of the QCPB method the relationship between frequency and mass becomes non-linear for high particulate loadings. In addition, since particles are collected on the crystal by impaction, the instrument response can be affected by the sharpness of the collection efficiency and the extent of particle bounce and internal particle losses. Investigators (Daley, P. S. and Lundgren, D. A., Am. Ind. Hyg. Assoc. J. 36:518, 1975) found that the frequency change for a given incremental mass deposit on the sensor does not remain constant as the sensor becomes loaded, due to changes in particle collection patterns over time. As previously discussed, the TEOM.RTM. method presents two serious shortcomings that make it inappropriate for measuring particle mass or any of its semi-volatile constituents such as water, ammonium nitrate, and organics. First, the sample air is heated at 50.degree. C. to dry the particles. This can result in particle mass losses up to 60% (Hering, S. V., 6th Conference of the Intl. Soc. for Environ. Epid./4th Conference of the Intl. Soc. for Expos. Anal. (joint conference), abstract no 260, Research Park Triangle, N. C., Sep. 1994; Meyer et al., A&WMA/EPA Conference, eds. Chow, J. C. & Ono, D. M., Scotsdale, Ariz. and Pittsburgh, Pa., Jan. 1992). Second, although the monitor provides continuous measurements, it utilizes the same filter to collect particles over a long sampling period (one-week) during which semi-volatiles can be adsorbed or desorbed, depending upon changes in atmospheric concentrations and meteorological conditions. Similar artifacts in particle measurements are also expected to occur for integrated multi-hour filter samples that collect particles, although losses should be less pronounced because the air sample is not heated. Finally, a large number of studies have used sorbents downstream the particle filters to measure losses of semi-volatiles and have found that a large fraction of particulate matter is not retained by the filter. Therefore, it is not recommended that the same filter be used for collecting multi-hour samples.
A continuous ambient mass monitor (CAMM) apparatus has been developed at the Harvard School of Public Health (Abstract of presentation at conference entitled "Measurement of Toxic and Related Air Pollutants", Research Triangle Park, N. C., Cosponsored by the U.S. Environmental Protection Agency and the Air and Waste Management Association, May 7-10, 1995). This apparatus provides for the real time measurement of the amount of particulate matter in a gas and is based on the monitoring of the pressure drop across a porous membrane filter over a period of time. However, this method has been limited to the measurement of the mass of ambient fine particles (less than 2.5 .mu.m in diameter).